It is well known in the art that the resistance modulation of magnetoresistors can be employed in position and speed sensors with respect to moving magnetic materials or objects (see for example U.S. Pat. Nos. 4,835,467, 4,926,122, and 4,939,456). In such applications, the magnetoresistor (MR) is biased with a magnetic field and electrically excited, typically, with a constant current source or a constant voltage source. A magnetic (i.e., ferromagnetic) object rotating relative and in close proximity to the MR, such as a toothed wheel, produces a varying magnetic flux density through the MR, which, in turn, varies the resistance of the MR. The MR will have a higher magnetic flux density and a higher resistance when a tooth of the rotating target wheel is adjacent to the MR than when a slot of the rotating target wheel is adjacent to the MR. The use of a constant current excitation source provides an output voltage from the MR that varies as the resistance of the MR varies.
Increasingly more sophisticated spark timing and emission controls introduced the need for crankshaft sensors capable of providing precise position information during cranking. Various combinations of magnetoresistors and single and dual track toothed or slotted wheels (also known as encoder wheels and target wheels) have been used to obtain this information (see for example U.S. Pat. Nos. 5,570,016, 5,731,702, and 5,754,042).
A target wheel of interest in this regard, is the 24X target wheel (see for example U.S. Pat. No. 5,570,016). This wheel and its associated sensor utilize analog signals which are converted into a 24 bit digital signal that is repeated every 360 degrees of rotation of the wheel. Each bit represents a particular position of the wheel and adjacent bits are angularly separated by 15 degrees. Prior art uses of this wheel have utilized a single sensor incorporating two matched MRs with a dual track wheel or a dual sensor, each sensor incorporating two matched MRs, with a single track wheel.
What is needed is a method and apparatus whereby the position of the crankshaft can be obtained via bit encoding utilizing one sensor incorporating two matched MRs in conjunction with a simple single track target wheel that can be inexpensively manufactured as an integral part of the crankshaft or as a separate item to be installed later.